The present invention relates to an input device for an information terminal such as a personal computer (PC), and in particular, to an approach position input device to detect a position of, for example, a fingertip of an operator.
JP-A-2002-342033 describes an example of a user's input device of non-contact type for use with a personal computer or the like. To recognize information items such as positional information items of two or more points, a contour of an approaching object, and distance to the object, the input device includes a plurality of transmission electrodes, a transmitter, a plurality of reception electrodes, and a receiver to receive an alternating current (ac) flowing through the reception electrodes. The transmitter supplies an alternating current for transmission to each of the transmission electrodes. The reception electrodes are disposed not to be in contact with the respective transmission electrodes.
The input device includes a first capacitor equivalent circuit virtually formed at each intersection between the transmission electrodes and the reception electrodes. When a conductive object such as a fingertip of a user approaches the input device, a second capacitor equivalent circuit is virtually formed in parallel with an associated first capacitor equivalent circuit. Electrostatic capacity of the second capacitor equivalent circuit varies according to distance to the conductive object approaching the input device.
JP-A-2000-020229 describes another conventional example of the position detecting device. This device simplifies signal processing to improve noise resistance performance and operability and includes a voltage vibrating system. The system includes a sensor panel or a sensor conductor array, a shield plate, a signal process circuit, a ground, and a current. The position detecting device conducts a ground signal process for electric vibration equivalently received via electrostatic coupling from a conductor to be detected and transmits a result of the process via an isolator to a non-vibration system.
The input device described in JP-A-2002-342033 can measure distance between the input device and a fingertip or can detect an event in which a fingertip is in touch with the input device. However, the transmission electrodes (m in number; m is an arbitrary number) to receive an alternating current and the reception electrodes (n in number; n is an arbitrary number) to receive a signal are separated from each other. Therefore, it is necessary to acquire the signal (m×n) times to detect information of a contour of an object such as a fingertip approaching the input device and distance therebetween. As a result, the detection period of time required to detect the event becomes longer. Although precision of detection is improved by increasing the numbers respectively of the transmission and reception electrodes, the increase in the number of electrodes elongates the detection period of time. This results in a disadvantage that the operator feels a slow reaction or a long reaction time of the input device.
The position detecting device described in JP-A-2000-020229 can detect an event in which a finger approaches the sensor conductor array. However, since the device detects the event using electrostatic capacity, a signal level of either x-directional reception signal or y-directional reception signal changes depending on cases. Therefore, when an approach position is obtained using only the signal level of x-directional reception signal or y-directional reception signal, there exits a fear of erroneous detection since an environmental change is not taken into consideration.